Optical fibers are commonly used to convey laser light for a wide variety of purposes. It is not infrequent for a single fiber with a core diameter on the order of 400 microns to be intended to carry laser light pulses for actuation purposes. Examples are ordnance firing systems. The fibers generally have a glass core with a glass cladding. They are terminated at a coupling to the user device. Depending on the configuration of the hardware, for example bulkheads and other places where a discontinuity in the fiber is required, there will be additional couplings in the optical circuit.
The optical continuity of the fiber and of the joined fibers is of utmost concern. The triggering of essential ordnance could be totally frustrated by a broken fiber or by a faulty connector. There are, of course, fiber optic systems for other uses in which continuity is also of primary importance. In all of these it is common practice to conduct frequent reliability tests.
Conventional testing techniques utilize time-domain reflectometry (TDR). These systems operate by transmitting a short pulse of light through the fiber and detecting its reflection from a dichroic mirror at the end of the fiber or at the end of a series of fibers that are coupled together. The dichroic mirror is reflective to the frequency of the test pulse, but transmissive of the frequency of the light from a firing or a signal pulse. The short period of time it takes for the pulse to traverse the fiber in two directions is known, and the receiver will see two pulses spaced a short time apart. The first pulse is light scattered from the transmitted pulse. The second pulse is the reflection from the dichroic mirror at the end of the fiber. The pulses are fed into a high speed voltage comparator which determines the pass/fail level. The pulses may be logically separated from each other to detect only the reflected pulse.
The TDR technique is used successfully for single fibers, one system for each fiber. However, there are many installations in which two or more fibers are used. For example, some laser ordnance systems require the initiation of simultaneous events. Commonly this is done with the use of two or more fibers, one respective to each event. While a test system can be provided for each of them, this soon becomes an economic burden, and in airborne systems, is an unacceptable weight penalty.
There are three common techniques for testing the continuity of systems which utilize more than one fiber. One technique is to require that both or all fibers be the exact same length. Then the reflected pulses will sum together at the detector. Practical system variables such as connector losses render this technique impractical. A reliable pass/fail level cannot be determined.
A second technique is to require the fibers to be of sufficiently different length so that separate pulse reflections can be detected for each fiber. This seriously complicates the detection circuitry and places unnecessary constraints on system design.
A third technique is to use a different wavelength laser diode for each fiber. This technique presents substantial laser and logistic problems both as to the electrical system and as to suitable dichroic mirrors for responding to the multiple wavelengths.
However complicated and troublesome, these techniques have found active use, not because they are especially good, but because they have been the best state of the art. It is an object of this invention to provide a continuity test system in which a single set-up can be used to test a plurality of optical fibers, thereby to reduce the cost and complexity of such systems, and to provide a more reliable pass/fail level.
The test system according to this invention utilizes polarized light and selector means respective to a plurality of cables, responsive to polarized light to select which of the fibers is to be tested for continuity. The selection requires only the manipulation of one optical element.